Typical multi-layered vacuum super insulated cryogenic tanks utilize a pair of cylindrical inner and outer tanks that are arranged concentrically with the inner tank residing in an interior of the outer tank. There are multiple radiant heat shields, approximately 30-80, coiled around the inner tank between the inner and outer tanks. A high vacuum exists between the inner and outer tanks to further prevent heat transfer. This type of thermal insulation is called a multi-layered vacuum super insulation. These storage tanks are capable of storing fluids at cryogenic temperatures.
The inner tank is positioned within the outer tank so that the inner tank does not contact the outer tank and so that thermal conduction paths between the inner and outer tanks are minimized. To facilitate this positioning, the inner tank typically has a pair of closed end pipes welded on opposite ends of the inner tank that form closed end channels that extend into the interior of the inner tank. A pair of rods are positioned in the channels to support the inner tank within the outer tank. One of these rods is fixed in the axial direction to inhibit axial movement of the inner tank in the axial direction. The other rod is loose in the axial direction to allow for displacement due to thermal expansion of the inner tank. The rods are designed so that the only contact between the rods and the inner tank is the interface between the ends of the rods and the ends of the channels. Opposite ends of the rods are attached to the internal surface of the outer tank. The rods, positioned on opposite ends of the inner tank, thereby support the inner tank within the outer tank.
To minimize the conductive heat paths, the rods are made from a carbon or glass fiber or other composite material. The carbon and glass fibers provide low thermal conductivity and help to isolate the inner tank from the outer tank. To further reduce the possibility of heat conduction between the inner and outer tanks, the rods can be made longer. That is, the length that the channels extend into the interior cavity of the inner tank can be increased, which decreases the volume of the inner tank, to allow for longer rods to be employed without increasing the dimensions of the outer tank. However, as the rods get longer, the bending force on the rods increases and a larger diameter rod is required to support the load over the longer distance. Additionally, the stress distribution inside the rods is not homogeneous thus requiring extra material when using rods regardless of the length. This in turn requires a larger surface area for the contact between the rods and the inner tank which increases the amount of heat being conducted through the rods and the resulting parasitic heat leak.
In other cryogenic storage tanks the inner tank is suspended within the outer tank by four rigid tensile rods or sticks at each end that extend radially to the outer tank. The rods are individually preloaded to suspend the inner tank within the outer tank. The use of four rods on each end statically overconstrains the inner tank thereby resulting in an undefined stress distribution due to the preload and when the inner tank is loaded with a fluid. The extra rods also provide extra parasitic heat leaks. Additionally, the individual preloading of each rod slows down production of such cryogenic storage tank thus increasing production cost.
Accordingly, it would be advantageous to provide an apparatus and method for supporting the inner tank within the outer tank that has a reduced intrusion on the inner tank and/or limiting the conductive heat paths between the inner and outer tanks. Additionally, it would be advantageous if such an apparatus and method facilitated mass production of the storage tanks. Furthermore, it would be advantageous if the apparatus and method were not statically over constrained thereby allowing determinable preloading and loading of the components of the apparatus.